tediousness vs dullness

tediousness and dullness both are nouns.

Word	Noun	Adjective	Verb	Adverb
tediousness	Yes	No	No	No
dullness	Yes	No	No	No

As nouns, dullness is a hypernym of tediousness; that is, dullness is a word with a broader meaning than tediousness:

tediousness (noun)	dullness (noun)
dullness owing to length or slowness	lack of sensibility
	without sharpness or clearness of edge or point
	the quality of lacking interestingness
	a lack of visual brightness
	the quality of being slow to understand